The present invention is in the field of direct current vehicle lighting systems, and more particularly, 42 volt systems.
Automotive vehicles increasingly require greater direct current (DC) electrical power. Vehicle electrical systems must handle ever greater loads, while at the same time meeting industry demands for lighter vehicles. At the same time, more wire is needed to supply all of the electricity-using components on the vehicle. Weight considerations mandate smaller gauge wire. Line loss considerations with low voltage (12V) power favor heavier gauge wire.
One known approach for supplying greater power to a vehicle electrical system without increasing wire thickness is to initially supply power at a much higher voltage, for example 42V. Theoretically, with the same current demand, the power from a 42V system is three times that of a 12V (actually 14V) system.
However, automotive headlights, brake lights, parking lights and the like prefer 12V power due to the unique resistance characteristics of the incandescent lamp filaments. The resistance of a filament at 42V is nine times higher than its resistance at 12V, or the square of the increase in power. This means that to operate properly, 42V lamps should have longer and/or thinner filaments than 12V lamps. The problem is that long, thin filaments are brittle and their optical quality is generally unfavorable.
One prior art approach to running 12V vehicle appliances such as lamp bulbs at 12V from a 42V power supply has been to use what is commonly referred to as a xe2x80x9ccentralized architecturexe2x80x9d. An exemplary centralized 42V architecture system is shown in FIG. 1. The vehicle alternator 10 keeps a 42V battery 11 charged. Battery 11 supplies 42V current to a first 42V line 13 and a DC/DC converter 15 which converts the 42V current to 12V and recharges a 12V battery 17. 12V lamp-using lights are then supplied with 12V current from battery 17 via lines 17a-17d. 12V filament-type bulbs are accordingly supplied with a steady flow of 12V current, for example at headlamps 18a, 18b and tail lights 18c, 18d. 
The problems with 42V/12V hybrid systems such as that shown in FIG. 1 include the requirement of two separate batteries in the vehicle, which greatly increases weight and cost. Additionally, DC/DC converters of the type illustrated at 15 are expensive, and can add considerable cost to a vehicle.
A less expensive, but still unsatisfactory, alternative 42V architecture is shown in FIG. 2. Alternator 10 still supplies a 42V battery 11 and keeps it charged. Battery 11 can supply straight 42V current along line 13 to 42V-using components. Instead of a DC/DC converter and separate 12V battery, however, distributed 12V lines to filament-type bulb using components such as head lamps and tail lights are each supplied with their own pulse width modulation (PWM) generators 16. PWM generators 16 comprise constant frequency, PWM generators of a commercially available type and are generally much less expensive than a DC/DC converter. The PWM generators give a pulsed, constant frequency output at 42V per pulse which, when averaged over time constitutes a roughly 12V supply to each 12V-using component. The manner in which pulse width modulation is used to couple and decouple the battery voltage to devices such as a lamp to provide an average DC voltage less than the actual battery voltage is generally known. See for example, U.S. Pat. No. 4,841,198 to Wilhelm. Devices for carrying out pulse width modulation are also known and commercially available, as will be recognized by those skilled in the art.
Despite the advantages of pulse width modulation in a distributed 42V architecture such as that shown in FIG. 2, pulse width modulated current creates problems with standard filament-type automotive lamp bulbs. PWM current operated at a low frequency results in higher efficiency at the lamp bulb (and resulting relatively brighter light), but a short life expectancy. High frequency PWM current extends the life of the lamp filament, but results in a low efficiency (relatively dim) light.
The present invention is an adaptive, variable frequency, pulse width modulated apparatus and method for supplying variable frequency and variable duty cycle power from a higher voltage source to a lower voltage electric load such as one or more lamps.
According to one aspect of the invention, an apparatus is provided for controlling the application of DC power to an electrical load in an automotive electrical system comprising a vehicle battery having a substantially higher voltage than is desired for direct application to the load. The system further comprises a pulse width modulation (PWM) circuit connected between the battery and the load to lower the applied voltage from, for example, 42V to, for example, 12V-14V, through appropriate duty cycle adjustment. In accordance with the apparatus aspect of the invention, further means are provided monitoring the condition of the battery and producing an output signal having a value with an indication of the condition of the battery; i.e., either good or bad. The circuit further comprises means for connecting the output signal of the monitor to the pulse width modulation circuit to vary the operating frequency thereof according to the value of the output signal.
In general, a higher operated rating frequency is selected when the battery condition is good and a lower operating frequency is selected when the battery condition is less than good.
According to a second, method aspect of the invention, a method of energizing an electrical load in a motor vehicle comprises the steps of supplying electrical power from a battery to a load through a pulse width modulatable switch, operating the switch at a duty cycle which decreases the battery voltage to a desired applied voltage, monitoring battery condition, and varying the frequency of the switch modulation based on the monitored battery condition so as to select a lower frequency of pulse width modulation whenever battery condition indicates the need to conserve battery power.
The apparatus and the method of the present invention utilize pulse width modulation to couple and decouple the battery voltage to vehicle electrical devices, such as lamps, to provide an average DC voltage, such as 12V, from the actual battery voltage of 42V. The apparatus and method automatically switch the pulse width modulator between high frequency and low frequency modes of operation corresponding respectively to an ordinary working mode and an energy saving mode depending upon the condition of the battery input and output current levels. This enables the present invention to extend a vehicle lamp""s life through use of the ordinary higher frequency mode as much as possible during normal operation of the battery, or to extend the battery life by use of the energy saving lower frequency during certain critical operating conditions when the load power demand or load current drawn from the battery exceeds the current supplied to the battery. In this later instance, a lower frequency is supplied to the pulse width modulator to reduce the load on the battery.